1. Field of the Invention
The present invention relates generally to a method for increasing signal coupling between pads of chips engaged in capacitive proximity communication (PxC). More specifically, the present invention relates to a method for increasing signal coupling, where a high permittivity dielectric material is directly deposited atop the pads to enable confinement of electric fields within pad gaps of the chips.
2. Background Art
With scaling of processor technology to future generational requirements, the pressing need for increased serial data rate over many communication channels to off-chip memory is not eased by the inability of electrical signaling to drive off-chip structures efficiently due to mismatch between on-chip and off-chip bandwidths. Thus, the ability of processors to exploit advancements in computational power, clock frequency, transistor count, multi-core architectures and the like is limited. One way to circumvent the aforementioned limitation is to employ PxC as a means to signal between processor and memory.
PxC is a form of wireless communication between chips, with the possible chip-to-chip coupling being capacitive, inductive or optical. Capacitive coupling may be employed to provide a high-bandwidth and high density channel between two chips placed face-to-face and extremely close to each other such that the transmitter circuits of one chip align with the receiver circuits of the other. An input/output (I/O) technology using the capacitive coupling of PxC can scale with on-chip features unlike traditional area ball bonds, leading to a largely improved I/O density. Thus, PxC with capacitive coupling offers the advantages of a hundred-fold increase in bandwidth density with a simultaneous three-fold reduction of on-chip power dissipation over traditional off-chip I/O signaling, thereby aiding utility in applications with power hungry, I/O limited multi-chip systems.
FIG. 1 shows two chips (110, 120) placed close to each other in a capacitively coupled PxC mode. Each chip includes transmitter (114, 122) and receiver circuits (112, 124), where a transmitter circuit of one chip (114, 122) communicates with the receiver circuit (112, 124) of the other chip by way of a metal plate on one chip and the metal plate on the other chip forming a capacitor. The metal plates of the transmitter circuit (114, 122) are depicted as shaded rectangles and the metal plates of the receiver circuit (112, 124) are depicted as non-shaded rectangles. A metal plate on the transmitter side is driven by the transmitter circuit (114, 122) and a metal plate on the receiver side drives the receiver circuit (112, 124).
When the two chips (110, 120) are moved close to each other, the surfaces of the dielectric and passivation layer touch each other, thereby allowing for excellent capacitive coupling between the metal plates due to their proximity. Also, the sizes of the transmitter and receiver structures may be reduced, allowing for reduced parasitic capacitances and huge power savings. The need for electrostatic discharge (ESD) protection devices is dispensed with as the metal plates lie under the passivation layer. This contributes to an additional reduction in capacitance and power consumption.
FIG. 2 shows a typical cross-section of transmitter 212 and receiver 222 pads of two chips (210, 220) involved in capacitive PxC. Communication between the transmitter 212 and receiver 222 pads is affected by three capacitances in positive and negative ways. The desired capacitance, i.e., the signal capacitance Cs, is the capacitance between the transmitter 212 and the receiver 222 pads. The unwanted capacitance due to coupling between adjacent transmitter 212 and receiver 222 pads is the noise capacitance Cn. As shown in FIG. 2, guard rings 224 may be provided between receiver 222 pads to suppress crosstalk between individual pads. However, an unwanted parasitic capacitance Cp may have to be factored into received signal calculations. This parasitic capacitance Cp may arise due to coupling between receiver 222 pads and guard rings 224, coupling between receiver 222/transmitter 212 pads and the ground plane, and/or intrinsic capacitance. Thus, these capacitances depend on inter-pad distance and inter-chip distance. Conservation of charge deems that the received signal, Vr, is dependent on the capacitances, and may be obtained from example Equation (1) as follows.(Cs+Cn+Cp)Vr=VsCs,  (1)
where Vs is the source signal. The received signal, Vr, as a function of capacitances, may, therefore, be expressed as:
                              V          r                =                                            C              s                        ⁢                          V              s                                            (                                          C                s                            +                              C                n                            +                              C                p                                      )                                              (        2        )            
Assuming equal dimensions w×l of two plates, equal plate thickness of t, and a separation between plates d, the capacitance C between two plates may be expressed as:
                              C          =                                                    ɛ                d                            ⁢                              (                                  w                  -                                      t                    2                                                  )                            ⁢                              (                                  l                  -                                      t                    2                                                  )                                      +                                          πɛ                ⁡                                  (                                      l                    +                    w                                    )                                            ⁢                              f                ⁡                                  (                                                            d                      2                                        ,                    t                                    )                                                                    ,                            (        3        )            
where ∈=∈0∈r is the permittivity of the medium between the two plates, ∈0 being the permittivity of free space and ∈r being the dielectric constant of the medium between the two plates, and f(d,t) is given by:
                              f          ⁡                      (                          d              ,              t                        )                          =                  1                      [                          ln              ⁡                              (                                  1                  +                                                            2                      ⁢                      d                                        t                                    +                                                                                                              2                          ⁢                          d                                                t                                            ⁢                                              (                                                                                                            2                              ⁢                              d                                                        t                                                    +                          2                                                )                                                                                            )                                      ]                                              (        4        )            
Using the method of images, Equation (3) is obtained by modeling the fringing fields of a rectangular micro-strip line over a ground plane as field lines due to a ground plane between two plates. Equation (3) does not account for the capacitance due to the four corners of the plates, which however is very small, but accounts for both the fringing fields and the parallel plate component. Equation (3) may be used to model the capacitance between transmitter 212 and receiver 222 pads, or any two finite pads separated by a small distance.
The crosstalk problem that arises in PxC may be due to fringing fields emanating from the length and width of the transmitter 212 pad not being coupled to the receiver 222 pad above and below the transmitter 212 pad (a receiver 222 pad above is not shown in FIG. 2). The crosstalk capacitance Cct1 may be modeled by subtracting from the capacitance when the receiver 222 pad is infinite the capacitance when the receiver 222 pad is of the same dimensions as the transmitter 212 pad, and may be expressed as:
                              C          ct                =                              πɛ            ⁡                          (                              l                +                w                            )                                ⁢                      (                                          f                ⁡                                  (                                      d                    ,                    t                                    )                                            -                              0.5                ⁢                                  f                  ⁡                                      (                                                                  d                        2                                            ,                      t                                        )                                                                        )                                              (        5        )            
The crosstalk problem may also have a contribution from two adjacent receiver 222 pads forming a parallel plate capacitor between each other, where the thickness of the pad contributes mainly to the capacitance Cct2, which may be expressed as:
                                          C                          ct              ⁢                                                          ⁢              2                                =                      ɛ            ⁡                          (                                                tl                                      (                                          p                      -                      w                                        )                                                  +                                  Af                  ⁡                                      (                                                                                            (                                                      p                            -                            w                                                    )                                                2                                            ,                      w                                        )                                                  +                                  1.47                  ⁢                  l                                            )                                      ,                            (        6        )            
where p is the center to center distance between adjacent pads, and A may be expressed as:
                    A        =                  0.5          ⁢                                          ⁢                      π            ⁡                          (                              t                +                l                            )                                ⁢                      (                          1              -                              0.0543                ⁢                                  w                                      (                                          p                      -                      w                                        )                                                                        )                                              (        7        )            
Typically, field lines from receiver 222 pads are not only coupled to adjacent receiver 222 pads but also coupled to the transmitter 212 plates below (not shown in FIG. 2). Therefore, the capacitance Cct2 reduces by a fraction, and may be expressed as:
                              C                      ct            ⁢                                                  ⁢            2                          =                              ɛ            ⁡                          (                              d                                  p                  -                  w                  +                  d                                            )                                ⁢                      (                                          tl                                  (                                      p                    -                    w                                    )                                            +                              Af                ⁡                                  (                                                                                    (                                                  p                          -                          w                                                )                                            2                                        ,                    w                                    )                                            +                              1.47                ⁢                l                                      )                                              (        8        )            
Misalignment is another problem in PxC. FIG. 3 shows a one-dimensional misalignment between two plates 310 and 320 in the direction of the width w. w1 is the width of receiver 310 plate that overlaps with a transmitter plate 320, whose width of non-overlap is w2. Thus, w can be expressed as:w=w1+w2  (9)
The capacitance Cma between two misaligned parallel plates 310 and 320 may be approximately expressed as:
                              C          ma                =                  ɛ          ⁡                      (                                                                                w                    1                                    ⁢                  l                                d                            +                              π                ⁢                                                                  ⁢                                  w                  1                                ⁢                                  f                  ⁡                                      (                                                                  d                        2                                            ,                      t                                        )                                                              +                              π                ⁢                                                                  ⁢                                  lf                  ⁡                                      (                                                                                            0.25                          ⁢                                                      (                                                                                          d                                2                                                            +                                                              w                                2                                2                                                                                      )                                                                                              ,                      t                                        )                                                                        )                                              (        10        )            
With the rapid changing status quo of Silicon technology, PxC requires chip separation on the order of 10 microns, which would shrink to smaller levels with future generation technology. In order to ensure high fidelity transmission of data streams with very low power and ultra-high density, gap requirements of a few microns and even sub-micron gaps may be the order of the day. Typical low-cost chip manufacturing processes achieve tolerances that are no better than a few mils (1 mil=25.4 microns).
FIG. 4 shows the plot of signal levels in flinging fields that are potential contributors to crosstalk in a dense PxC array as a function of chip separation for different pad dimensions. The x-axis denotes chip separation 410 in microns, and the y-axis signal levels 420 expressed as a percentage. Three different pad dimensions are chosen, viz. 1 micron pads 430, 10 micron pads 440, and 50 micron pads 450. FIG. 4 clearly demonstrates that when pad sizes are reduced to convenience high-density packaging, the signal levels coupled from pad to pad decrease with increase in chip separation 410. Alternatively, signal levels 420 in fringe fields increase with increase in chip separation 410.